In a TV system, luminance and chrominance components are mixed in a composite TV signal transmitted on a single channel. The components are separated in TV sets prior to the recovery of the primary color signals for the display. However, for broadcast TV systems, this process has imposed a substantial limitation on the horizontal resolution available in TV sets. Signal intermodulation arising in the bands occupied by the chrominance subcarrier signal have produced degradations in the image known as cross-color and cross-luminance. The first causes a display of false colors to be superimposed on repetitive patterns in the luminance image, while the second causes crawling dot patterns, primarily visible around colored edges. To improve the definition of TV sets, the signal mixture between luminance and chrominance is substantially reduced using comb filters.
Referring to FIG. 1, a typical comb filter 30 may comprise, for example, a delay circuit 32 that delays the input composite signal by one line-scan interval equal to 64 .mu.s for the PAL and SECAM systems or equal to 63.555 .mu.s for the NTSC system. In broadcast type video signals, the chrominance component is band-limited in the vertical direction. In this case, as shown in FIG. 2, the chrominance subcarrier signal shifted by 180.degree. from line to line (according to the NTSC standard) has essentially the same amplitude on subsequent horizontal lines 1 and 2. Accordingly, an adder 34 that adds the delayed signal to the input provides a luminance output nearly devoid of chrominance content. Conversely, a subtractor 36 that subtracts the delayed signal from the input, produces a chrominance output similarly devoid of luminance. When the output luminance and chrominance signals are used to recover the cross-color signals, the cross-color and cross-luminance effects are largely removed.
Unlike the broadcast type video signals, computer generated images can have very sharp color transitions from line to line. As shown in FIG. 3, the chrominance content of the computer generated video signal on line 1 is completely different from the chrominance on line 2. Accordingly, if computer generated video signals are supplied to the comb filter 30, it would introduce chrominance leftovers from line 1 into the chrominance and luminance signals of the subsequent line 2. As a result, highly visible artifacts would appear to an observer as one or more dashed lines below sharp color transitions. For example, in the case of a thin horizontal line with a bright red saturated color on a black background, the artifacts will be represented by up to 5 dashed red lines under the original line. The intensity of the dashed lines decreases as their distance from the original line increases.
The growth in multi-media computer applications that require the images that are generated by portable computers having small screens to be displayed on large screens of external TV sets, for example, on a large viewing screen of a projection TV set, creates a need for interfacing a computer to a TV set. However, as discussed above, the comb filter in a TV set causes the artifacts to be superimposed over the displayed image on a TV screen. When displayed on a large screen TV, such as for a multi-media presentation, these artifacts will become particularly noticeable and objectionable.
To prevent the artifacts from being formed, it would be desirable to disable the comb filter in the luminance/chrominance separator of the TV set when the computer generated images are supplied to the TV set. In this case, the TV set separator will revert to the bandpass/bandstop filtering strategy that is currently used to separate luminance and chrominance of a video signal supplied from a video tape recorder (VTR). VTR video signals have specific time-base distortions recognizable by a TV set that switches to a VTR mode of operation in response to these distortions.